The coating of thermoplastic polymer substrates with moisture resistant barrier coating compositions to provide impermeability to gases such as oxygen, and liquids, is known. See, e.g., U.S. Pat. No. 3,282,729, which describes applying a water solution of poly(vinyl alcohol) (PVOH) and trimethylolphenol to a thermoplastic polymer substrate at an elevated temperature. Also, for example, U.S. Pat. No. 5,073,419 refers to a film composite comprising a linear low density polyethylene film having a PVOH coat of a thickness of about 0.1 to about 3 mils. U.S. Pat. No. 5,487,940 refers to a metallized polymeric film structure including an oxygen barrier and a moisture barrier. The oxygen barrier includes cross-linked PVOH and the moisture barrier is preferably metallized oriented polypropylene or polyethylene.
Similarly, U.S. Pat. No. 4,254,170 refers to bioriented polyester hollow bodies wherein one wall of a preform is coated with an aqueous composition consisting of at least two incompatible polymers, one of which is a water soluble PVOH and the other, a polymer latex with low water sensitivity.
U.S. Pat. No. 5,384,192 refers to a structure comprising an inorganic oxide substrate having an organic polymer layer. There is an adhesion promoting layer between the substrate and organic polymer layer. One component of the adhesion layer is poly(vinyl phenol), also known as poly(hydroxystyrene).
U.S. Pat. No. 5,192,620 refers to a metallized film composition comprising a polymer substrate having a surface which is modified by an adhesion promoting agent, in which the modified surface is provided with a skin layer of PVOH. The PVOH skin layer is applied by a solution coating process. The skin layer has a metal layer directly thereon.
U.S. Pat. No. 5,491,023 refers to a metallized film composition comprising a polymer substrate having a surface which is modified by an adhesion promoting agent; the modified surface is provided with a skin layer of PVOH. The PVOH skin layer is applied by an extrusion process. The skin layer has a metal layer directly thereon.
Despite the wealth of art in barrier coatings, currently available polymeric films do not accommodate the need in the market for longer shelf life of packaged food. Further, many of these products (e.g., films coated with aluminum) are not microwave-safe, or are not readily disposable (e.g., films coated with poly(vinylidene chloride), and thus fail to satisfy environmental concerns.
There exists a need in the art for additional compositions and methods which provide improved barrier coatings to polymeric products, e.g., films, bottles, etc.
In one aspect, the present invention provides a method for providing a barrier coating to a polyolefin substrate, preferably a film. The method involves the steps of applying to the polymeric substrate a primer composition containing a random vinyl copolymer comprised of repeat units, wherein at least 75% of said copolymer repeat units contain side groups having hydroxyl moieties. The primer layer is then dried. The third step of the method involves subsequently applying a waterborne barrier coating solution which forms a dry inorganic barrier layer over the dried primer layer.
In another aspect, the present invention provides a barrier-coated polyolefin article.
Other aspects and advantages of the present invention are described further in the following detailed description of the preferred embodiments thereof.
The present invention provides a method of promoting the wetting of waterborne barrier coating solutions on polyolefin substrates and adhesion of resulting inorganic barrier layers to polyolefin substrates by applying a selected primer composition to the substrate before application of the barrier coating solution. The improved adhesion of dried inorganic barrier layers is manifested in improved vapor barrier performance.
A primer composition according to the present invention contains a random vinyl polymer comprised of repeat units, (i.e., derived from xe2x80x9cvinylxe2x80x9d monomers like ethylene, propylene, vinyl acetate, vinyl phenol, etc.) wherein at least 75% of said polymer repeat units contain side groups having hydroxyl moieties. Desirably, vinyl polymers include poly(vinyl alcohol) and poly(para-hydroxystyrene).
In one embodiment, the vinyl polymer useful in the primer is a poly(vinyl alcohol) (PVOH). There are numerous PVOH variants available commercially and/or known in the art. For example, such PVOH variants vary in characteristics such as molecular weight and percentage hydrolyzed. Poly(vinyl alcohol) is derived from poly(vinyl acetate) by hydrolysis of the acetate function. Typically a fraction of the acetate functions are left intact to impart different properties. The percent hydrolysis refers to the fraction of acetate groups which have been hydrolyzed to hydroxyl moieties. Desirably, the average molecular weight distribution of the PVOH polymers useful in the primer composition is between about 50,000 and about 185,000 and the PVOH is more than about 75% hydrolyzed.
The PVOH useful in this invention may be modified by randomly replacing some of the vinyl alcohol groups with vinyl butyral groups as depicted by the formula: 
wherein n, p, and r denote the mole fractions of the polymer repeat units and the sum of n, p and r is 1. To achieve satisfactory wetting of barrier coating solutions on these primer layers it is preferable that r is at least 0.75. Desirably, the average molecular weight of such a modified PVOH is between about 40,000 and 100,000.
The PVOH variants may be dissolved in a suitable solvent (e.g. water, isopropanol, or mixtures thereof), so that the primer composition is characterized by a solids level of between 0.1% and 10% by weight. When the polymer in the primer composition is PVOH, preferably, the solvent useful in the primer composition is water. However, other suitable solvents for PVOH may be readily selected by one of skill in the art.
It is well known that PVOH solutions support the growth of microorganisms. To prevent this, it is conventional practice to optionally add at least one biocidal agent to the coating solution. Desirable biocides include those with the active ingredients 5-chloro-2-methyl-4-isothiazolin-3-one, 2-methyl-4-isothiazolin-3-one, or 1-(3-chloroallyl)-3,5,7-triaza-1-azoniaadamantane chloride. A representative biocide that was found not to interfere with the performance of dried primer layers in coated articles of the present invention is Kathon(copyright) LX (Rohm and Haas) biocide.
Another embodiment of a vinyl polymer useful in the primer composition is a linear or branched poly(para-hydroxystyrene) (PHS) or a variant thereof. There are a number of PHS variants available commercially and/or known to the art. For example, such PHS variants vary in characteristics such as molecular weight, impurities and degree of branching. While the examples below employ several variants of poly(para-hydroxystyrene), (branched, linear, and partially converted to phenoxide salts) it is anticipated that other PHS variants, for example copolymers in which the PHS mole fraction is at least 0.75, will work similarly in the primer composition and method of this invention.
Desirably, the primer composition containing PHS is characterized by a solids level of between 0.1% and 10% by weight. Preferably where the polymer in the primer composition is a PHS variant, the solvent is a dilute aqueous solution of alkaline metal hydroxide. For example, a lithium (or sodium or potassium) hydroxide concentration of 0.1 N may be used to prepare a 1 weight percent solution of PHS. In this solvent PHS is partially converted to an alkali metal (e.g., lithium) phenoxide salt. The resulting linear or branched random copolymer has the formula: 
where M+ is a monovalent cation (e.g., Li+)and the sum of the mole fractions, n and p, is 1. The mole fraction, p, of the phenoxide salt can be controlled by adjusting the concentration of alkaline metal hydroxide.
Dynamic contact angle experiments have demonstrated that use of PHS solutions in 0.1 N LiOH to form a primer layer on corona-treated, biaxially-oriented polypropylene (BOPP) films provides a surface that is completely wetted by water. Further, metal copolysilicate coatings have been found to wet such a primed surface extremely well, and barrier layers produced thereby give good vapor barrier performance. For solutions with lesser amounts of base, a cosolvent is required to achieve complete PHS dissolution. Such a cosolvent may be readily selected by one of skill in the art. One exemplary suitable cosolvent is isopropanol. Alternatively, PHS may be simply dissolved in an alcohol, such as ethanol, propanol, butanol, isopropanol (2-propanol) and similar alcohols, alone or in mixtures. Neutral PHS solutions prepared using high purity PHS (e.g. Hoechst Celanese Electronic Grade), yield neutral PHS primer layers that exhibit time-dependent and pH-dependent wettability. Solutions with pH above 12 completely wet neutral PHS primer layers. PHS primer solutions prepared using lower purity PHS (e.g. Hoechst Celanese Polymer Grade) yield primer layers that are equally wettable at any pH above 7.
Optionally, the primer solution, whatever the identity of the primer polymer, further contains one or more surfactants to reduce surface tension. It was found that surfactants were unnecessary when applying primers by spin-coating; however wetting requirements were more severe when applying primers by roll coating methods, e.g. reverse gravure coating. Suitable surfactants may be readily selected by one of skill in the art. The surfactant selected should possess a critical micelle concentration sufficiently low to ensure a dried primer coating uncompromised by residual surfactant. Preferably, the surfactant is selected from the group consisting of acetylenic diols (e.g., such as those provided commercially by Air Products) and alkyl ethoxylates (such as those provided commercially by, among others, Hoechst Celanese). Of the former group a preferred surfactant is the Dynol(copyright) 604 surfactant; of the latter group a preferred surfactant is the Genapol(copyright) UD050 surfactant. The amount of surfactant added to the primer composition will depend on the particular surfactant selected, but should be the minimum amount of surfactant that enables adequate wetting of the primer solution on the polyolefin substrate. For example, typical surfactant amounts can be about 0.1% by weight of an acetylenic diol or an alkyl ethoxylate. Because Dynol(copyright) 604 surfactant is sparingly soluble in water, it has been observed that primer solutions containing 0.1% Dynol(copyright) 604 surfactant have droplets of undispersed surfactant floating on the surface of the primer solution. It was observed that this causes formation of scum around the meniscus of the coating solution in containers such as jars, drums, and solution holding tanks. In preferred primer solutions, this is alleviated by adding a second surfactant, preferably Genapol(copyright) UD050 surfactant, to improve the dispersion of the Dynol(copyright) 604 surfactant.
The method of the invention is useful with a variety of waterborne, inorganic coating compositions, such as those described below, that act as barriers to gases, vapors and aromas.
By xe2x80x9cwaterbornexe2x80x9d is meant coatings that are applied from solutions in which the solvent is primarily water, but which may contain smaller amounts of cosolvents such as, but not limited to, isopropanol.
The term xe2x80x9cvaporxe2x80x9d implies a liquid at partial pressure, such as water vapor. The term xe2x80x9cgasxe2x80x9d includes oxygen, nitrogen, carbon dioxide and others. xe2x80x9cAromaxe2x80x9d includes those materials which bear a fragrance, for example, menthol and others. For simplicity, as used herein, the term xe2x80x9cvapor barrierxe2x80x9d can be interpreted to mean a barrier to gases and aromas as well as traditionally defined vapors.
Similarly as used herein, the term xe2x80x9csolutionxe2x80x9d is interpreted to include colloidal dispersions and suspensions. By xe2x80x9ccolloidal dispersion or suspensionxe2x80x9d is meant any dispersion or suspension of particles in liquid, the particles being of a size greater than molecular scale that do not settle out. Generally, the particle size in a suspension or dispersion of this invention are from about 10 to about 50,000 Angstroms. xe2x80x9cCoating solutionxe2x80x9d as used herein is meant a liquid containing dissolved or suspended solids that do not settle out and which is used to apply said solids to a substrate.
In one embodiment the inorganic, waterborne coating contains an alkali metal polysilicate, such as sodium polysilicate, potassium polysilicate, or lithium polysilicate or mixtures thereof.
In another embodiment the coating solution contains a copolysilicate, i.e., a mixture of two different alkali metal polysilicates. In a preferred embodiment the barrier coating solution contains a copolysilicate of lithium and potassium represented by the formula, (Li2O)x(K2O)1xe2x88x92x(SiO2)y, wherein y is greater than 4.6 if x is less than 1 or x is greater than 0.5 if y is between 1 and 10.
Another desirable barrier coating contains a selected layered silicate (e.g. the delaminated vermiculite MicroLite(copyright) product, of W. R. Grace) dispersed in a solid matrix of an alkali metal polysilicate (or mixtures thereof), such that the weight percentage of the layered silicate in the dried barrier coating layer ranges from 1% to 99%.
Specific formulations of suitable barrier coatings for use in the method and compositions of this invention are described in more detail in the examples below.
Advantageously, in the practice of the method of the invention, the primer composition promotes good wetting of the subsequently applied coating solution to the substrate and good adhesion of the dried inorganic barrier layer. The latter advantage is manifested in improved vapor barrier performance of coated articles prepared according to the present method.
A. The Substrate
The method of this invention is particularly well suited for use on polymeric substrates such as polyolefins, particularly polyethylene, polypropylene, copolymers thereof, and cycloolefinic copolymers (COC) such as a copolymer of ethylene and norbornene [U.S. Pat. No. 5,087,677]. Typically, polypropylene films are biaxially-oriented, depending on the customer requirements. The articles coated by this method include, without limitations, polymeric films and sheets, rigid and semi-rigid containers, and other surfaces. Especially preferred articles for coating according to the method of this invention are films, bottles, plastic containers, jars, blisterpacks and lidstocks, made of the foregoing polymers. In a particularly preferred embodiment, the articles are films or bottles used for food storage.
The polymeric articles to be coated by the primer and coating composition according to this invention may be previously untreated. Usually, the polymeric article, such as a film or bottle, is first plasma treated to improve wetting by the primer solutions and adhesion of the dried primer layer. Alternatively, the polymeric article may be corona-treated by the industry-wide corona discharge treatment method. Other applicable surface treatments that may precede application of the primer layer are flame treatment and chemical oxidation or etching. Optionally, after applying the primer solution and drying, the dried primer layer may be plasma treated, corona treated, flame treated, or chemically oxidized or etched before applying a barrier coating solution. Alternatively, the article may bear on at least one surface or side, a heat seal layer. Examples of such heat seal layers are an ethylene-propylene copolymer or ethylene-propylene-butylene terpolymer.
Exemplary polyolefin substrates used in the examples below are the FND xx and SCM xx grade biaxially-oriented polypropylene (BOPP) films produced by Trespaphan GmbH. The number, xx, refers to the film thickness in micrometers; thus FND 30 is a 30 xcexcm (or 1.2 mil) thick BOPP film. These films are designed to be metallized by thermal or electron-beam evaporation of aluminum. FND and SCM grade BOPP films are three-layer laminates with thin heat-sealable layers on both sides. One side of the film is corona treated at the factory to a surface energy of 36-39 dynes/cm to improve adhesion of aluminum. Supplemental corona treatment, immediately before applying a primer solution, was found to be beneficial even for these films that were corona treated at the factory. Similar commercially available polypropylene films that will be suitable in the present invention include AQS, MT BASE, and MVT BASE films (AET Packaging Films). These all have a heat seal layer on one side only and, in the case of AQS, a high energy treated surface for waterborne coatings. These are all intended to be coated on the side opposite from the heat seal layer.
B. Application of the Primer
In a preferred embodiment, the primer solution is applied to provide a dried layer thickness of between about 10 to about 50 nm primer composition on the substrate. Thicker layers offer satisfactory, but not superior, performance, and are therefore less preferable on the basis of cost. The primer may be applied by any technique known to those of skill in the art. These techniques include, without limitation, roll coating, spray coating, and dip coating techniques. Conventional roll coating techniques include, but are not limited to, rod, roll, reverse roll, forward roll, air knife, knife over roll, blade, gravure and slot die coating methods. General descriptions of these types of coating methods may be found in texts, such as Modern Coating and Drying Techniques, (E. Cohen and E. Gutoff, eds; VCH Publishers) New York (1992) and Web Processing and Converting Technology and Equipment, (D. Satas, ed; Van Nostrand Reinhold) New York (1984). Three-dimensional articles may be coated by spray coating or dip coating. The method of application is not a limitation on the present invention, but may be selected from among these and other well-known methods by a person of skill in the art.
Preferred primer layers applied according to the present method are substantially continuous, i.e. very little of the polyolefin is exposed to the barrier coating solution. Dewetting of the primer solution from the substrate before or during drying causes voids in the primer layer and, subsequently, voids in the barrier layer. This yields a finished article having inferior (but possibly, for some applications, satisfactory) vapor barrier performance. In extreme cases, dewetting of primer solutions was observed to yield a polyolefin substrate that was largely uncovered, but merely decorated with particles of primer material. Atomic force microscopy may be used to confirm that dried primer layers are substantially continuous. Dewetting may be minimized by increasing treatment of the polyolefin substrate before applying the primer solution, increasing surfactant concentration, or increasing the viscosity of the primer solution. The latter is readily accomplished in PVOH primers by using high molecular weight PVOH grades and increasing the percent PVOH solids in the primer solution.
The primer layer is allowed to dry before application of the waterborne, inorganic coating layer. A conventional coating thickness of the selected waterborne inorganic oxide coating solution is applied over the primer layer, i.e., typical coating thicknesses as used in the absence of primer, such as between about 100 and about 500 nm on the surface of the substrate. The application of the coating solution may be performed as described above for application of the primer composition.
After coating the article with a barrier coating solution, the resulting product must be dried at a selected temperature at or above room temperature. The selection of the drying temperature depends on the desired time for drying; that is, accelerated drying times may be achieved at elevated temperatures which would not be necessary if a longer time period for drying was acceptable. However, one of skill in the art can readily adjust the oven temperature and exposure as desired. The performance of the dried barrier coating is insensitive to the drying temperature over the range 25-200xc2x0 C. An advantage of the present method is that both the primer and barrier coatings can be dried at low temperature ( less than 100xc2x0 C.) which is necessary when roll-coating polypropylene film.
As one example, a resulting article (e.g., a BOPP film, 1 mil in thickness) coated according to the method of this invention is typically provided with an oxygen transmission rate (OTR) less than 50 cm3/[m2 day atm] at 23xc2x0 C. and 50% relative humidity. In favorable instances an OTR of about 20 cm3/[m2 day atm] at 23xc2x0 C. and 50% relative humidity can be achieved. This performance has been more reproducibly achieved in pilot scale reverse gravure coating experiments than in spin-coating experiments.
Significantly improved performance can be achieved if the dried barrier coating is covered with a protective top-coat layer. The top-coat may be either a thin (typically, but not necessarily, 1-10 xcexcm thick) coating or a laminated film. Thin top-coatings may be applied by a variety of coating methods: roll coating, spray coating, dip coating. Laminates may be prepared by melt-extrusion lamination over the barrier coating or by adhesive lamination of a second film. Coated articles (BOPP films) prepared according to the present method, when provided with protective top-coat layers, have achieved OTRs of about 10 cm3/[m2 day atm] at 23xc2x0 C. and 50% relative humidity. The top-coat further provides improved flex resistance, i.e. retention of vapor barrier performance after flexing, and moisture resistance, i.e. retention of vapor barrier performance at high relative humidity.